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Why does the IUD (intra uterine device) contain copper?

Why copper?

The IUD formerly consisted of a soft and elastic, plastic material. Its effectiveness lay solely in preventing the implantation of a fertilized egg cell in the uterine wall. The Chilean gynecologist Zipper discovered that a small amount of copper prevents sperm from fertilizing the egg cell. This is why the modern IUD is entwined with a copper thread: It is a safer method because fertilization is thereby prevented.

Use of the IUD also therefore leads to a very low frequency of pregnancies occuring outside the uterus (ectopic and ovarian pregnancies).